A hemispherical resonant gyro generally comprises a hemispherical resonator of silica, comprising a shell with a pole held on a support rod, the shell including an annular edge defined by a hemispherical inside surface and a hemispherical outside surface that are concentric so that the shell presents constant thickness from the pole to the annular edge. The models for such surfaces are well known and they can therefore be machined with great precision. Nevertheless, a shell of constant thickness presents the drawback of having a modal mass, i.e. an effective mass, that is less than 20% of the total mass of the shell. This means that it is not possible to take full advantage of the very low damping of silica. Increasing the modal mass has the advantage of decreasing parasitic damping effects, in particular the damping that results from the metallization; of decreasing the relative effect of geometrical defects in machining; and of increasing the quantity of momentum of the vibration.
To increase modal mass, proposals have been made to increase the thickness of the shell while keeping it constant. Nevertheless, a uniform increase in shell thickness leads to an increase in resonant frequency in the same proportion as the increase in modal mass, such that the overall performance is not satisfactory.
Furthermore, for reasons that are independent of modal mass, proposals are made in document FR-A-2 792 772 to increase the thickness of the shell in the vicinity of its annular edge. It turns out that that configuration makes it possible to increase modal mass with little increase in the resonant frequency of the resonator. Nevertheless, in the above-specified document, the increase in the thickness of the shell along the edge is obtained by changing the shape of the inside and outside surfaces defining the shell, thus making the shell more complex to machine and increasing the risk of machining defects.